With the aid of real-time sonography, visual observation of the dynamic behavior of tissues in response to externally applied mechanical stress (palpation) can make available diagnostically useful information related to tissue elasticity and high level tissue structure, particularly the connectivity of different tissue structures, which affects the tissue's shear properties. In the case of breast examination, for example, this technique of "ultrasound palpation" has become for some diagnosticians a routine part of the sonographic test procedure. The elastic properties of the tissue are often discussed in terms of "mobility" and "compressibility", both of a suspicious region and of its immediate surroundings. The information so obtained can enhance the ability of ultrasound imaging to contribute to detection and differential diagnosis of tumors.
The use of digital ultrasonic echo processing methods, in combination with either controlled externally induced tissue displacement or with naturally occurring cardiac motion, to quantify this motion, image it and measure a tissue elastic modulus has also been of interest. This approach has involved the investigation of a wide variety of both processing techniques and methods of applying the force to the tissue. In general, the force which produces the tissue displacement may be applied manually or automatically.
All currently available real-time ultrasound imaging systems display their image data using a coordinate system which is defined by an origin whose reference is fixed with respect to the transducer. Thus, whether a visual inspection of real-time images or a digital processing method is employed for elasticity studies, if the tissue displacement is produced by moving the ultrasound probe (which may be a transducer array or a mechanical scan head), there is a global image transformation (translation and/or rotation) which is due to the varying relationship between the fixed reference point and the tissue structures. If the movement is large enough, as is often the case with manually induced motion, this effect may interfere with the ability of a visual or numerical analysis to identify the local motion (and therefore elastic) anomalies which comprise the diagnostic information.
Other problems generated by the conventional approach to scan conversion for real-time ultrasound imaging systems are that skilled operation is required to avoid "transducer shake" (the ultrasonic equivalent of camera shake) during detailed study of stationary tissue structures, especially in high resolution scanning modes, and that the effort required to hold the transducer still and/or visually compensate for the moving scene may result in operator fatigue. With the current trend toward very high frequency (20-150 Mhz), very high resolution imaging of intravascular and superficial structures, the problems of transducer shake are likely to increase in severity to a point where they will compromise the resolution potentially available for these devices. Another manifestation of transducer shake is the "flash artifact" on color Doppler images. Finally, full advantage cannot be taken of image enhancement procedures, such as temporal averaging and compounding, since translations and rotations due to transducer motion will blur the average imaged information.
Prior to the introduction of high quality, real-time ultrasonic imaging using hand-held automatic scanning systems, ultrasonic imaging was conducted using static B-scanners. The images, which were not real-time, were generated by manually moving a single element transducer to steer and/or translate the ultrasound beam. The images were displayed via a scan converter using display coordinates that were located in the tissue. It was possible for a skillful operator to produce images which contained global motion corrected elastic information. See, for example, Ch. M. Gros et al, Senologia, Juin 1978, No. 2, pps. 3-14. However, because of the static nature of the images, it was not possible to separate time and spatial information. Both were incorporated in a complex manner within a single image.
Three-dimensional imaging and visualization systems, utilizing a two-dimensional ultrasound imaging system and a position and orientation sensing system to construct a three-dimensional image, have been proposed. N. Shinozuka et al in WFUMB '94 and WFS -94 Abstracts, Ultrasound Med. Biol., Vol. 20, page S267 describe the use of an accelerometer/velocity sensing type of device to sense the position and orientation of a transducer. D. Rotten et al in BMUS Bulletin, November 1992, British Medical Ultrasound Society, London, pps. 18-23 refer to the use of a mechanical sensing arm to provide the position and orientation of a transducer. A. Moskalik et al in Abstracts, Ultrasonic Imaging and Tissue Characterization Symposium, 1994, Ultrasonic Imaging, Vol. 16, pps. 47-48 describe how information as to the position and orientation of the transducer may be derived from the image data itself. P. R. Detmer et al in Ultrasound and Medicine in Biology, Vol. 20, No. 9, pps. 923-936, 1994 describe the use of a "pulsed magnetic field" type of space tracker for sensing the position and orientation of the transducer in a three-dimensional ultrasound imaging system. M. D. Handschumacher et al in J. Am. Coll. Cardiol., Vol. 21, No. 3, Mar. 1, 1993, pps. 743-753 describe the use of a spark gap type of space tracker to sense the position and orientation of a transducer in a three-dimensional ultrasound imaging system.